Mossbauer studies are proposed for the following enzymes: (1) hydrogenases from Clostridium pasteurianum W5 and from sulfate-reducing bacteria, (2) nitride reductase from Thiobacillus denitrificans, and (3) sulfite reductases from T. denitrificans and sulfate-reducing bacteria. Hydrogenase is capable of activating molecular hydrogen and many micro-organisms use it to metabolize H2. Understanding the hydrogenase mechanism could be a key step toward catalysis of hydrogen production from water and solar energy, a potentially important alternate energy source. Nitrite reductase reduces nitrite to nitric oxide, while sulfite reductase catalyzes the 6-electron reduction of sulfite to sulfide. These reductions of nitrite and sulfite are, respectively, essential steps in nitrate and sulfate respirations. Despite the importance of these enzyme's biological functions, their prosthetic groups are poorly understood. The proposed studies should reveal physical properties and structural information pertinent to the prosthetic groups of these enzymes; information essential to the understanding of their respective mechanisms. The results will be correlated with EPR, NMR, ENDOR, and optical data, and hopefully will provide a sound experimental basis for detailed physical models of the prosthetic groups.